1. Field of the Invention
The present invention relates to a method and an apparatus of driving an aircraft power generator at a constant-speed. More specifically, the present invention relates to a method of driving an aircraft power generator by the output power of the engine of an aircraft at a fixed operating speed regardless of the engine speed, and a constant-speed driving apparatus for carrying out the method.
2. Description of the Related Art
In an aircraft, such as a passenger jet airplane, a generator is driven by the rotational output of the main engine to generate AC power (three-phase, 115 V, 400 Hz) for operating electrical devices of the lighting system, the air conditioning system, the anti-icing system and the like. The thrust of a jet engine is adjusted by properly adjusting the engine speed and, generally, the engine speed changes according to the change of the thrust. Therefore, a constant-speed drive (CSD) capable of adjusting the variable input engine speed to a fixed rotational speed for driving the generator is necessary to generate AC power of a specified frequency, such as 400xc2x17 Hz specified in MIL-STD-704E, by the variable rotational output of the jet engine. Integrated drive generators (IDGs) are prevalently used as aircraft power generators. The integrated drive generator is constructed by combining a constant-speed drive and a generator.
Such integrated generators are disclosed in Japanese Patent Publications Nos. 7780/1980, 7781/1980 and 7782/1908 which comprise a differential gear driven by the engine, a displacement hydraulic pump with motor, and a control circuit operated by a governor to change the volume of the displacement hydraulic pump. The displacement hydraulic pump is an oil-hydraulic pump. The generator is driven at a fixed rotating speed by controlling the output rotational speed of the differential gear by the oil-hydraulic pump and a hydraulic motor.
However, since the oil-hydraulic pump and the hydraulic motor are a piston pump and a piston hydraulic motor, the previous constant-speed drive has the following drawbacks.
(1) Since the piston pump and the piston hydraulic motors are provided with pistons that reciprocate in cylinders, seizure is liable to occur, joints are subject to fatigue failure and abrasion and are unsatisfactory in reliability. Incidentally, whereas demanded MTBUR (Mean Time Between Unscheduled Removal) is 15,000 hr, the mean of actual takedown times is 5,000 hr or below.
(2) Since the constant-speed drive uses hydraulic power as principal power, the power transmission efficiency of the constant-speed drive is as low as the order of 65%, which increases the fuel consumption of the aircraft. In a 150-passenger medium airplane, the constant-speed drive increases fuel consumption by about 1%.
(3) The complicated mechanism of the previous constant-speed drive deteriorates reliability, and increases weight and costs.
(4) Since the principal part of the previous constant-speed drive is a reciprocating mechanism, the rotating speed cannot be increased any further, and further weight and size reduction cannot be expected.
Thus, it is preferable to use a continuously variable speed transmission, such as a traction drive, capable of operating at a high rotating speed and has a life that can be exactly estimated. Since the continuously variable speed transmission comprises rotary components, the life thereof can be exactly estimated by a method similar to that of estimating the life of bearings. The employment of the continuously variable speed transmission, such as a traction drive, improves greatly the drawbacks in the mechanism including the oil-hydraulic pump and the hydraulic motor.
If the transmission mechanism of the constant-speed driving apparatus is so formed as to transmit all the power necessary for driving the generator, for example, only by a traction drive, the constant-speed drive, which meets dimension and weight requirements, might be unable to secure necessary durability thereof. Furthermore, the effect of the employment of the traction drive in improving efficiency is not satisfactory because the power transmission efficiency of the traction drive is on the order of 85%.
Mechanisms intended to provide an automotive continuously variable speed transmission having an extended life and capable of operating at an increased efficiency are disclosed in Japanese Laid-Open Publications Nos. 169169/1989 and 63147/1999. Each of those previously proposed mechanisms comprises a toroidal traction drive and a planetary gear in combination. However, those mechanisms cannot be used on aircraft for the following reasons.
(1) In some operating condition, 100% of power is transmitted to the traction drive and hence the traction drive must have heavy and large construction. If the traction drive is formed in dimensions not greater than those required of traction drives suitable for use on aircraft, the traction drive is unable to secure a necessary life.
(2) Power circulates in the planetary gear of the known mechanism. Therefore, power is consumed uselessly and dimensions of the planetary gear are unnecessarily large. The planetary gear is unable to secure a necessary life if the same is formed in dimensions not greater than those required of planetary gears for use on aircraft.
(3) Since the planetary gear is disposed outside the traction drive in the known mechanism, the mechanism has a large overall size.
(4) The constant-speed drive is unable to meet requisite conditions in order to be used on aircraft with respect to its weight, dimensions and life for the foregoing three reasons.
(5) Although an automotive continuously variable speed transmission is designed so that its gear ratio is controlled to make the engine operate at an engine speed at which the engine is able to produce a desired driving force and to operate at a minimum fuel consumption rate, the continuously variable speed transmission for driving the aircraft power generator must be designed to drive the generator at a fixed operating speed regardless of the variation of the engine speed.
The present invention has been made in view of the foregoing problems in the related art. It is therefore an object of the present invention to provide a method of driving an aircraft power generator installed on an aircraft at a constant-speed using constant-speed driving apparatus capable of operating at high efficiency with high reliability, having an extended life, formed in compact construction, and capable of driving the aircraft power generator for the stable generation of AC power of a fixed frequency according to the operating condition of the aircraft.
Another object of the present invention is to provide a constant-speed driving apparatus for carrying out the foregoing method of driving an aircraft power generator.
According to a first aspect of the present invention, a constant-speed driving method of driving an aircraft power generator installed on an aircraft by an engine of the aircraft at a constant-speed includes the steps of: splitting an output rotational driving power of the engine into a first split power and a second split power; transmitting the first split power to a continuously variable speed transmission that transmits the first split power by a shearing resistance of a fluid; transmitting the second split power to a differential planetary gear system; transmitting an output power of the continuously variable speed transmission to the differential planetary gear system to combine the first split power and the second split power in the differential planetary gear system; and absorbing a variation of a rotating speed of the output rotational driving power by the continuously variable speed transmission to adjust an output rotating speed of the differential planetary gear system to a constant speed.
Preferably, the second split power is transmitted to one of a sun gear, a planetary carrier and a ring gear of the differential planetary gear system; and the output power of the continuously variable speed transmission is transmitted to another one of the sun gear, the planetary carrier and the ring gear.
Preferably, the second split power is transmitted to one of the sun gear and the ring gear; and the output power of the continuously variable speed transmission is transmitted to another one of the sun gear and the ring gear.
Preferably, the sun gear, the planetary carrier and the ring gear are rotated in a same direction.
According to a second aspect of the present invention, a constant-speed driving apparatus for driving an aircraft power generator installed on an aircraft by an engine of the aircraft at a constant-speed comprises: a power splitting mechanism that splits an output rotational driving power of the engine into a first split power and a second split power; a continuously variable speed transmission to which the first split power is transmitted, the first split power being transmitted via the continuously variable speed transmission by a shearing resistance of a fluid; and a differential planetary gear system to which the second split power and an output power of the continuously variable speed transmission are transmitted, the first split power and the second split power are combined in the differential planetary gear system. A variation of a rotating speed of the output rotational driving power is absorbed by the continuously variable speed transmission to adjust an output rotating speed of the differential planetary gear system to a constant speed.
Preferably, the differential planetary gear system includes a sun gear, a planetary carrier and a ring gear; the second split power is transmitted to one of the sun gear, the planetary carrier and the ring gear; and the output power of the continuously variable speed transmission is transmitted to another one of the sun gear, the planetary carrier and the ring gear.
Preferably, the second split power is transmitted to one of the sun gear and the ring gear; and the output power of the continuously variable speed transmission is transmitted to another one of the sun gear and the ring gear.
Preferably, the sun gear, the planetary carrier and the ring gear are rotated in a same direction.
Preferably, the continuously variable speed transmission comprises a toroidal traction drive.
Preferably, the toroidal traction drive is a double-cavity toroidal traction drive; and the differential planetary gear system is disposed coaxially with the continuously variable speed transmission.
Preferably, the double-cavity toroidal traction drive comprises output disks which are disposed on opposite sides of the differential planetary gear system, respectively, and an output shaft which supports the output disks; and the output shaft of the double-cavity toroidal traction drive also serves as a sun gear of the differential planetary gear system.
Preferably, the differential planetary gear system comprises a planetary carrier and a ring gear having an external gear; and an output of the planetary carrier is transmitted through the external gear of the ring gear.
Preferably, a speed change ratio of the continuously variable speed transmission decreases with an increase of an engine speed of the engine with a result of a deceleration; and the speed change ratio increases with a decrease of the engine speed with a result of an acceleration.
According to a third aspect of the present invention, a control method of controlling the constant-speed driving apparatus as defined above comprises the steps of: controlling the continuously variable speed transmission so that a speed change ratio of an output rotating speed of the constant-speed driving apparatus to an input rotating speed of the constant-speed driving apparatus is fixed when an engine speed of the engine is below a predetermined low rotating speed; and controlling the continuously variable speed transmission so that the output rotating speed of the constant-speed driving apparatus is fixed when the engine speed of the engine is in a predetermined engine speed range which is above the predetermined low rotating speed.
Preferably, the output rotating speed of the constant-speed driving apparatus is measured by a rotating speed measuring device; and a deviation of the output rotating speed measured by the rotating speed measuring device from the input rotating speed is used as a speed change command signal to be given to the continuously variable speed transmission.
Preferably, a signal produced by adding a change rate of the input rotating speed and the deviation together is used as the speed change command signal to be given to the continuously variable speed transmission.
According to a fourth aspect of the present invention, a controller for controlling a constant-speed driving apparatus as defined above comprises: device for controlling the continuously variable speed transmission so that a ratio of an output rotating speed of the constant-speed driving apparatus to an input rotating speed of the constant-speed driving apparatus is fixed when an engine speed of the engine is below a predetermined low rotating speed, and device for controlling the continuously variable speed transmission so that the output rotating speed of the constant-speed driving apparatus is fixed when the engine speed of the engine is in a predetermined engine speed range which is above the predetermined low rotating speed.
Preferably, the output rotating speed of the constant-speed driving apparatus is measured by a rotating speed measuring device; and a deviation of the output rotating speed measured by the rotating speed measuring device from the input rotating speed is used as a speed change command signal to be given to the continuously variable speed transmission.
Preferably, a signal produced by adding a change rate of the input rotating speed and the deviation together is used as the speed change command signal to be given to the continuously variable speed transmission.
According to a fifth aspect of the present invention, an aircraft power generating system comprises: an aircraft power generator; a constant-speed driving apparatus as defined in claim 5; and a housing containing the constant-speed driving apparatus and the aircraft power generator.
The constant-speed driving apparatus and the method of driving an aircraft power generator at a constant-speed according to the present invention improve the efficiency and the reliability of the aircraft power generator more effectively than the previous constant-speed drive or method employing the oil-hydraulic pump and the hydraulic motor.
Since the rotational driving power for driving the generator is split to the continuously variable speed transmission utilizing the shearing resistance of a fluid and the power splitting shaft of the differential planetary gear system, the life of the speed changing means can be extended and power transmission efficiency can be improved.
Since the constant-speed driving apparatus has the foregoing features and is capable of high-speed driving, the constant-speed driving apparatus can be formed in lightweight, compact construction.
A control method according to the present invention is capable of controlling the constant-speed driving apparatus for operation matched with the operating characteristic of the aircraft.
In a preferred embodiment, the differential planetary gear system and the continuously variable speed transmission are disposed coaxially, and the sun gear of the differential planetary gear system serves also as the output member of the continuously variable speed transmission. Therefore, the differential planetary gear system can be installed in a greatly reduced space and the constant-speed driving apparatus can be easily formed in lightweight, compact construction.